Q.Give reasons for the anomalous behaviour of Li. Write any four points of similarities between Li and Mg.

Lithium behaves anomalously compared to the rest of Group 1 mainly because it is exceptionally small and has a very high charge/size ratio (polarizing power); this same property gives it a strong 'diagonal relationship' resemblance to magnesium, and the two share several distinctive chemical behaviours.

Reasons for the anomalous behaviour of lithium:

1. Exceptionally small atomic and ionic size — Li is by far the smallest alkali metal atom/ion, much smaller than Na, K, Rb, Cs.
2. High charge density / high polarizing power — because Li⁺ is so small for its +1 charge, it polarizes neighbouring anions strongly (Fajans' rules), giving Li compounds noticeably more covalent character than the ionic compounds of the rest of Group 1.
3. Absence of vacant d-orbitals — unlike heavier alkali metals, Li has no accessible d-orbitals in its valence shell, limiting its bonding/coordination behaviour and contributing to differences in reactivity and complex formation.
4. Highest ionization enthalpy and highest electronegativity in the group — because of its small size, Li holds its valence electron more tightly than the other alkali metals, giving it somewhat less metallic/more covalent-leaning character.
5. High hydration enthalpy of Li⁺ — the small Li⁺ ion is very strongly hydrated in aqueous solution, which affects properties like its standard electrode potential (making Li the strongest reducing agent in water among alkali metals, despite having the highest ionization enthalpy) and the solubility/behaviour of its salts.

These properties together mean Li often behaves more like Group 2 metals (especially Mg) than like its own group members — this is called the diagonal relationship.

Four similarities between Li and Mg:

1. Oxide formed on burning in air: Both Li and Mg burn in oxygen to give only the simple normal oxide (Li2O and MgO respectively) — unlike Na, K, etc. (which form peroxides/superoxides).
2. Covalent chlorides: Both LiCl and MgCl2 have significant covalent character (unlike the chlorides of the rest of Group 1, which are essentially fully ionic); both are soluble in organic solvents like ethanol/pyridine. …